Provided is a rotary tool bit such that a soft core body having hardness lower than that of a hardened layer formed by applying plasma carburization to the entire surface of the bit, is disposed inside said hardened layer, and that thereby while wear resistance is improved, local stress concentration can be prevented, with the internal soft core body used as a cushion. In other words the rotary tool bit (1) is such that there is formed the hardened layer (2), which is formed by applying plasma carburization to the entire surface (S) of the bit, and that the soft core body (3) having hardness lower than that of the hardened layer (2) is made to exist inside the hardened layer (2), with the result that it became possible to improve wear resistance.